Self-limiting electrical triggering for initiating fracture of frangible glass

ABSTRACT

A transient electronic device includes electronic elements (e.g., an SOI- or chip-based IC) and a trigger mechanism disposed on a frangible glass substrate. The trigger mechanism includes a switch that initiates a large trigger current through a self-limiting resistive element in response to a received trigger signal. The self-limiting resistive element includes a resistor portion that generates heat in response to the trigger current, thereby rapidly increasing the temperature of a localized (small) region of the frangible glass substrate, and a current limiting portion (e.g., a fuse) that self-limits (terminates) the trigger current after a predetermined amount of time, causing the localized region to rapidly cool down. The frangible glass substrate is engineered such that a stress profile produced by the rapid heating/cooling of the localized region generates an initial fracture force that subsequently propagates throughout the glass substrate, whereby sufficient potential energy is released to powderize the electronic elements.

RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 15/220,164, filed Jul. 26, 2016, which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention is based upon work supported by DARPA under Contract No. HR0011-14-C-0013. Therefore, the Government has certain rights to this invention.

FIELD OF THE INVENTION

This invention relates to transient electronics, and in particular to trigger devices and triggering methods utilized to initiate the fragmentation of frangible glass substrates in transient electronic assemblies.

BACKGROUND OF THE INVENTION

Large area sensing is critical for a variety of military, ecological and commercial interests and has historically been served through the use of centralized long-range sensors. However, rapid improvements in miniaturization of electronic systems have significantly improved the capabilities of small sensor devices. These micro-sensors have the potential to create “large N” distributed networks with advantages in operational adaptability, non-traditional sensing modalities that are only possible with close proximity, increased sensitivity and knowledge extraction through networked intelligence.

While distributed network systems have remarkable promise, their realistic use is limited by risks associated with their accumulation in the environment, detection and defeat, and exploitation due to inability to maintain positive control (unlike centralized long-range sensors).

The phrase “transient electronics” refers to a relatively new family of electronic devices that disappear (disaggregate and disperse) within a set period of time, making them ideally suited for distributed network systems. Conventional transient electronic systems typically rely on the use of soluble substrates and electronic materials (such as silk). When placed into solvent (typically water), these conventional substrates and electronics slowly dissolve into solution. As such, a distributed network system made up of conventional transient electronic devices can be expected to “disappear” over a relatively short amount of time (e.g., after periodic rainfall).

Although the conventional transient electronic approaches achieve the goal of causing the electronics to “disappear” after use, the long dissolution period required to achieve complete disaggregation and dispersal make the conventional approaches unfit for discrete (e.g., military) applications that require rapid and complete disaggregation upon command. Moreover, early conventional approaches utilize materials that were not compatible with existing integrated circuit fabrication and assembly techniques, requiring the development of new IC fabrication processes at significant cost.

More recently, a new type of transient electronic device was introduced in which functional circuitry fabricated using well-known low-cost fabrication techniques (e.g., CMOS or SOI) and disposed on a frangible glass substrate that, when subjected to a small initial fracture force, underwent complete disaggregation (shattering) in a manner that releases sufficient potential energy to also cause disaggregation of the functional circuitry. The transient event (i.e., disaggregation) was controlled by a trigger mechanism configured to generate the required initial fracture force by way of applying resistive heating, a chemical reaction or a localized mechanical pressure to the frangible glass substrate in response to a suitable trigger signal. An issue with some triggering approaches is that they may require a substantial electronic system to provide a particular current or voltage pulse in order to provide the necessary conditions to initiate fragmentation. For example, resistive heating type trigger mechanisms may require a pulse shaping circuit configured to generate and transmit a shaped current pulse from a power source to the resistive heating element in order to generate initial fracture.

What is needed is a simple and reliable trigger mechanism and triggering method for initiating the powderization of a frangible glass substrate in a transient electronic device that reliably achieves complete, on-command disaggregation of the electronic circuitry formed thereon in response to an electronic trigger signal.

SUMMARY OF THE INVENTION

The present invention is directed to a simple and reliable trigger mechanism and associated triggering method for reliably initiating powerderization of functional circuitry (electronic elements) in a transient electronic device by way of generating a localized thermal pulse (i.e., rapid heating followed by rapid cooling) using a self-limiting circuit arrangement that avoids the need for current pulse shaping circuitry and/or complicated connection arrangements, thereby avoiding the problems associated with conventional approaches.

According to an aspect of the invention, the trigger mechanism includes a self-limiting resistive element that is at least partially formed on a frangible glass substrate. The frangible glass substrate is constructed substantially entirely from a glass material having an associated melting point temperature using known techniques such that it undergoes complete disaggregation (shattering) in response to an applied initial fracture force. The electronic elements (e.g., a silicon-on-insulator-based (SOI-based) IC, a chip-based IC, or thin-film electronics patterned directly onto the frangible glass substrate) and the self-limiting resistive element of the trigger mechanism are fixedly attached to one or both surfaces of the frangible glass substrate, with the electronic elements typically dispose over a first region of the frangible glass substrate, and the self-limiting resistive element disposed over a localized (i.e., relatively small) second region of the frangible glass substrate. With this arrangement, the localized thermal pulse generated by the self-limiting resistive element produces a stress profile caused by rapid heating above the glass' melting point temperature and then cooling in the localized region located adjacent (e.g., below) the self-limiting resistive element. By configuring the trigger mechanism in the manner described below, the resulting stress profile generates the desired initial fracture force in the localized region, thereby initiating the complete disaggregation of the frangible glass substrate, along with the electronic elements disposed thereon.

According to another aspect of the present invention, the self-limiting resistive element includes a current-limiting portion (e.g., a fuse, timer-based or thermistor-based circuit) that independently controls (i.e., without requiring an externally-generated control signal) an amount generated heat by way of terminating the flow of a trigger current through the self-limiting resistive element after a predetermined amount of generated heat is transmitted into the localized region of the frangible glass substrate. In an exemplary embodiment, the current-limiting portion is implemented by a fuse structure that is configured to melt/break after a predetermined amount of heat is generated by the self-limiting resistive element, thereby producing an open circuit that terminates further heat generation by way of preventing further trigger current flow through the self-limiting resistive element. Moreover, by configuring the self-limiting resistive element in the manner described below such that heat rapidly dissipates from the localized region of the frangible glass substrate after termination of the trigger current, the self-limiting resistive element provides a simple and reliable mechanism for generating the desired thermal pulse and resulting stress profile that produce the initial fracture force in the localized region of the frangible glass substrate.

According to another aspect of the present invention, the self-limiting resistive element facilitates electronic control of the triggering event using very simple addressing electronics, thereby minimizing system complexity. That is, because the self-limiting resistive element eliminates the need for current shaping electronics, the trigger mechanism is able to achieve complete, on-command disaggregation of the electronic circuitry formed on the frangible glass substrate using a simple switch element that is controllable (i.e., actuated to generate the trigger current) by way of an electronic trigger signal to selectively couple the self-limiting resistive element to a direct-current (DC) power source (e.g., a battery). Using this arrangement, the switch element is turned on at the beginning of a transient event in response to the electronic trigger signal, thereby initiating a large direct current (i.e., the trigger current) to flow from the DC power source through the self-limiting resistive element, whereby the self-limiting resistive element operates as described above to generate the thermal pulse resulting in the initial fracture force causing disaggregation of the frangible glass substrate. As such, in contrast to conventional resistive-heat-type trigger mechanisms that require pulse shaping circuity and complicated addressing arrangements, the triggering mechanism of the present invention is simple and reliable, thereby reducing manufacturing costs and complexity.

As mentioned above, characteristics of the thermal pulse (i.e., the rate and duration of heat generation that produces the rapid temperature increase) are entirely independently controlled by operation of the self-limiting resistive element (i.e., without requiring an externally-generated control signal). Specifically, the self-limiting resistive element is configured to generate resistive heat (i.e., by way of passing the large trigger current through one or more resistor structures) for a predetermined amount of time after actuation of the switch element, whereby a temperature of the localized region of the frangible glass substrate rapidly increases from a relatively low initial temperature (e.g., approximately 140° C.) to a high (first) temperature level temperature (e.g., approximately 220° C.) that is above the melting point temperature of the glass forming the frangible glass substrate. The self-limiting resistive element is further configured to independently control the amount of generated heat by way of terminating the flow of trigger current through the resistor element(s) at the end of the predetermined time period, whereby terminating the heat generating process causes rapid cooling of the localized region (e.g., from the high (first) temperature (e.g., 220° C.) to a lower (second) temperature (e.g., approximately 200° C.), e.g., in approximately one second) by way of heat dissipation from the localized region into surrounding substrate regions. The present inventors determined that a thermal pulse generated in this manner produces a stress profile that reliably generates an initial fracture force in the localized region having sufficient strength to produce subsequent propagating fracture forces that pass throughout the frangible glass substrate, causing “powderization” (i.e., disaggregation or fragmentation) of the frangible glass substrate into micron-sized particulates. When a transient electronic device is constructed in accordance with the present invention, sufficient potential energy is released from the disaggregating/fragmenting glass substrate to cause propagation of cracks from the glass substrate into the electronic elements and other device structures (e.g., the self-limiting resistive element) with sufficient force to entirely powderize these structures as well. Because the trigger mechanism produces the rapid heating/cooling stress profile using only a simple switch and a self-limiting resistive element (i.e., without requiring an external control signal or other circuitry capable of generating a shaped current pulse), the present invention provides a low-cost and reliable triggering mechanism for initiating powderization of electronic elements disposed on a transient electronic device.

According to an embodiment of the present invention, the self-limiting resistive element is entirely formed by a resistive material that is deposited and patterned directly onto a surface of the frangible glass substrate using a standard thin-film fabrication technique (e.g., photolithographic or inkjet printing). In alternative embodiments, the resistive material includes magnesium, copper, tungsten, aluminum, molybdenum or chrome, or a combination of one or more of these metals. Other suitable materials may also be used, provided the material is sufficiently conductive and adheres to the frangible glass substrate surface well enough that heat is transferred efficiently into the localized region during the rapid heating portion of the thermal pulse.

According to an embodiment of the present invention, the self-limiting resistive element includes one or more resistive portion connected in series with the current-limiting portion between two (first and second) terminals, with the first terminal is coupled by way of the switch element to the direct current (DC) power source and the second terminal connected to a suitable ground structure. In one embodiment, the resistive material mentioned above is patterned to form two relatively large resistor structures connected by a narrow fuse element, where the resistor structures are configured to heat up but remain coherent during the thermal pulse, and the fuse element is configured to melt and break at the end of the rapid heating (first) time period. The use of a fuse element to implement the current-control portion of the self-limiting resistive element provides a low-cost, simple and highly reliable structure for independently controlling the amount of heat generated during the rapid heating process, and reliably produces an open circuit condition (i.e., by way of melting/breaking) that terminates flow of the trigger current through the resistor structures at the start of the rapid cooling portion of the thermal pulse.

In a presently preferred embodiment, the resistive material utilized to form the self-limiting resistive element is patterned into a bowtie-type structure in which the two resistor structures including respective downward-tapered sections, and the fuse element is formed by a narrow section of resistive material connected between tapered ends of the downward-tapered sections. In one embodiment, the fuse element includes a straight rectangular structure extending between the tapered end, and in another embodiment the narrow resistive material section forming the fuse element is disposed in a substantially S-shaped pattern. In either case, a width W of the narrow resistive material section is greater than a thickness of the frangible glass substrate determines to induce reliable disaggregation. In a practical embodiment using a 0.25 mm thick frangible glass substrate, the narrow resistive material section forming the fuse element has a width of at least 0.3 mm. In contrast, resistor structures can have almost any size, although while larger resistors ensure reliable fragmentation, they also require more power and energy from the power source. Accordingly, the smallest possible resistor structures are preferred in order to minimize power and energy requirements.

According to an embodiment of the present invention, to facilitate remote (wireless) control over the transient electronic device, the transient electronic device further includes a sensor configured to detect a transmitted wave signal (e.g., a light wave signal, a radio frequency (RF), or an acoustic/sound signal), and configured to then generate the electronic trigger signal used to actuate the switch element. In an exemplary embodiment, a remote optical signal is detected by a photodiode (light wave sensor), whereby current through the photodiode causes a silicon controlled rectifier (switch element) to latch, which in turn couples a battery across the self-limiting resistor element, ultimately causing disaggregation (fragmentation) of the glass substrate and any included electronics. While remote actuation of the transient electronic device is achieved using an optical signal in this example, RF signals and acoustic wave signals may be utilized by replacing the photodiode with a radio-frequency wave sensor or an acoustic wave sensor. Moreover, other physical and chemical stimuli may also be utilized in conjunction with an appropriate sensor to initiate a fragmentation sequence. Similarly, while latching is achieved using a silicon controlled rectifier, other switch elements may also be utilized, such as a single MOSFET transistor or a multiple-element latch circuit.

According to other alternative practical embodiments, one or more of the switch element and sensor utilized to control operation of the triggering mechanism may be formed/mounted directly on the frangible glass substrate, formed on a semiconductor layer/die on which the electronic elements are formed, or disposed on an external printed circuit board to which the transient electronic device is attached. For example, in the practical embodiment described above, one or both of the photodiode/sensor and the silicon controlled rectifier (switch) may be implemented by patterning suitable materials directly onto the frangible glass substrate, or may be implemented using CMOS fabrication techniques on a semiconductor structure (e.g., IC chip or SOI layer) on which the electronic elements are formed and that is fixedly attached to the frangible glass substrate.

In additional alternative embodiments, the frangible glass substrate comprises one of a stressed glass substrate including stress-engineered tensile and compressive layers that are operably attached together, a thermally tempered glass substrate including laminated layers of different glass types having associated different coefficient of thermal expansion (CIE) values, and an ion-exchange treated glass substrate. In each case, the frangible glass substrate contains enough stored energy to generate self-propagating secondary fractures in response to an initial fracture force such that the glass substrate completely disaggregates (“powderizes”) into micron-sized particulates by way of a mechanism similar to that captured in a Prince Rupert's Drop. In alternative embodiments, the electronic devices are either fabricated directly onto the frangible glass substrate using standard silicon-on-insulator (SOI) fabrication techniques (i.e., such that the functional circuitry is implemented as an SOI integrated circuit structure), or are separately fabricated on an IC die (chip) that is then attached to the frangible glass substrate using anodic bonding. In either case, the released potential energy during disaggregation of the frangible glass substrate is sufficient to also powderize the electronic devices, along with the trigger mechanism and any other structures that might be disposed on the frangible glass substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where:

FIG. 1 is a top side perspective view showing a transient electronic device including a trigger mechanism disposed on a frangible glass substrate according to a simplified embodiment of the present invention;

FIG. 2 is a flow diagram showing a generalized method for triggering a disaggregation event on the transient electronic device depicted in FIG. 1;

FIGS. 3(A), 3(B), 3(C), 3(D), 3(E) and 3(F) are partial cross-sectional side views showing a portion of the transient electronic device of FIG. 1 at various moments during a disaggregation event;

FIG. 4 is a time-temperature graph illustrating an exemplary thermal pulse generated during the disaggregation event described with reference to FIGS. 3(A) to 3(F);

FIG. 5 is a time-voltage/current graph illustrating exemplary operating characteristics of a trigger mechanism utilized during the disaggregation event described with reference to FIGS. 3(A) to 3(F);

FIG. 6 is a partial perspective view showing the self-limiting resistive element of a triggering mechanism according to an exemplary practical embodiment of the present invention;

FIGS. 7(A) and 7(B) are top plan views showing self-limiting resistive elements having bowtie-type patterns according to alternative exemplary specific embodiments of the present invention;

FIG. 8 is a circuit diagram showing an exemplary trigger mechanism configured for remote actuation according to another embodiment of the present invention; and

FIGS. 9(A) and 9(B) are simplified perspective views showing transient electronic devices including remote actuation according to alternative specific embodiments.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention relates to an improvement in transient electronic devices. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, directional terms such as “upper”, “upward”, “lower”, “downward” and “over” are intended to provide relative positions for purposes of description, and are not intended to designate an absolute frame of reference. The terms “coupled” and “connected”, which are utilized herein, are defined as follows. The term “connected” is used to describe a direct connection between two circuit elements, for example, by way of a metal line formed in accordance with normal integrated circuit fabrication techniques. In contrast, the term “coupled” is used to describe either a direct connection or an indirect connection between two circuit elements. For example, two coupled elements may be directly connected by way of a metal line, or indirectly connected by way of an intervening circuit element (e.g., a capacitor, resistor, inductor, or by way of the source/drain terminals of a transistor). Various modifications to the preferred embodiment will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.

FIG. 1 includes simplified perspective views showing a transient electronic device 100 in a pre-transience “whole” state (i.e., device 100(t0) shown in the middle portion of FIG. 1) and a post-transience “powderized” state (i.e., device 100(t2) shown in the lower portion of FIG. 1). In the pre-transience state (e.g., immediately after production, during normal operation, and during portions of the transient event occurring prior to disaggregation), transient electronic device 100(t0) generally includes functional electronics 120 and a trigger mechanism 130 that are disposed over corresponding regions of frangible glass substrate 110. In the preferred embodiment depicted by device 100(t2) at the bottom of FIG. 1, in the post-transience “powerderized” state, the frangible glass substrate and any structures disposed thereon (i.e., functional electronics 120 and any portion of trigger mechanism 130 mounted on frangible glass substrate 110) are completely disaggregated (“powderized”) into micron-sized particulates 101 (i.e., have sizes ≤100 μm across).

In order to produce transient electronic device 100 such that it achieves the preferred post-transience “powerderized” state depicted in FIG. 1, it is necessary to manufacture or procure frangible glass substrates having the characteristics set forth below. Referring to the middle of FIG. 1, frangible glass substrate 110 is a wafer-like structure having opposing upper and lower surfaces 111 and 112, and is constructed using known techniques such that it undergoes complete disaggregation (shattering) in response to the initial fracture force generated/applied by trigger mechanism 130. In one specific embodiment, frangible glass substrate 110 includes at least one glass structure including a first glass material having a first coefficient of thermal expansion (CIE) value, and multiple second glass structures respectively including one or more different (second) glass materials respectively having a second CTE value, where the second CTE value is different from the first CTE value. For example, as indicated in the upper right bubble in FIG. 1, thermally tempered glass substrate 110A includes a glass structure 110A-1 disposed between two glass structures 110A-21 and 110A-22, where glass structure 110A-1 comprises a different glass material having a different CTE value than the glass material from which glass structures 110A-21 and 110A-22 are formed. Similarly, as indicated in the upper left bubble of FIG. 1, thermally tempered glass substrate 110B includes (second) glass structures 110B-21, 110B-22 and 110B-23 disposed in a (first) glass structure 110B-1, where glass structure 110B-1 comprises a different glass material than that of glass structures 110B-21 to 110B-23. The fabrication of such thermally tempered glass substrates is described in U.S. Pub. App. No. 2015/0358021-A1, which is incorporated herein by reference in its entirety. In another alternative specific embodiment, frangible glass substrate 110 comprises a stressed glass substrate including stress-engineered tensile and compressive layers that are operably attached together in the manner described in U.S. Pat. No. 9,154,138, which is also incorporated herein by reference in its entirety. In yet another alternative specific embodiment, frangible glass substrate 110 an ion-exchange treated glass substrate or interposer fabricated in the manner described in U.S. patent application Ser. No. 14/694,132 filed Apr. 23, 2015 and entitled “Transient Electronic Device With Ion-Exchanged Glass Treated Interposer” which is also incorporated herein by reference in its entirety. When implemented using any of the specific embodiments mentioned above, frangible glass substrate 110 contains enough stored energy to generate self-propagating secondary fractures in response to an initial fracture force such that, as depicted by device 100(t2) at the bottom of FIG. 1), frangible glass substrate 110 completely disaggregates (“powderizes”) into micron-sized particulates (i.e., ≤100 μm across) using a mechanism similar to that captured in a Prince Rupert's Drop. In addition, the released potential energy during disaggregation of frangible glass substrate 110 is sufficient to also powderize functional electronics 120 as also illustrated by device 100(t2). In one embodiment, the released potential energy also powderizes portions of trigger mechanism 130 and any other structures that might be disposed on frangible glass substrate 110.

Functional electronics 120 include electronic elements 122 fabricated on a semiconductor base layer 121 (e.g., an SOI layer or an integrated circuit (IC) die/chip) that is fixedly attached to upper surface 111 and dispose over a corresponding region 110-1 of frangible glass substrate 110. As set forth above, the main purpose of transient electronics is to provide a way to essentially entirely eliminate an IC or other electronic elements for purposes of protecting the environment or maintaining confidentiality (i.e., preventing unauthorized reverse engineering of the elements/circuit). A benefit of fabricating transient electronic device 100 on frangible glass substrate 110 is that this approach both facilitates forming functional electronics 120 using low cost manufacturing techniques, and facilitates reliable elimination of functional electronics 120 by way of causing disaggregation of frangible glass substrate 110. In a preferred embodiment, functional electronics 120 are fabricated by way of forming electronic elements 122 on a suitable semiconductor (base) layer 121 using existing IC fabrication techniques (e.g., CMOS), and electronic elements 122 are configured to perform a prescribed useful function (e.g., sensor operations) up until the transient event. In one embodiment, the semiconductor layer 121 is a silicon “chip” (die) upon which electronic elements 122 are fabricated, and then the semiconductor layer 121 is fixedly attached to glass substrate 110 using known die bonding techniques (e.g., anodic bonding or by way of sealing glass) that assure coincident powderization of electronic elements 122 with frangible glass substrate 110. In a presently preferred embodiment, functional electronics 120 includes electronic elements 122 configured to form an IC device using standard silicon-on-insulator (SOI) fabrication techniques (i.e., such that the functional circuitry is implemented as an SOI integrated circuit structure). In another embodiment, functional electronics 120 are fabricated on an IC die that is “thinned” (e.g., subjected to chemical mechanical polishing) before the bonding process.

As set forth above, trigger mechanism 130 is configured to generate an initial fracture force in frangible glass substrate 110 in response to an electronic trigger signal TS. According to the present invention, trigger mechanism 130 achieves this function using a self-limiting resistive element 140 and a switch element 150 that are connected in series between a battery (or other DC power source PS) and ground. As depicted in FIG. 1, at least a portion of trigger mechanism 130 is disposed on upper surface 111 over a second region 110-2 of frangible glass substrate 110 (i.e., away from functional circuitry 120), but may alternatively be formed on lower surface 112. Switch element 150 has a first terminal connected to battery PS, a second terminal connected to a first terminal 141-1 of element 140, and a control terminal operably disposed to receive electronic trigger signal TS, which may be generated by an optional sensor or other control circuitry (not shown). As set forth below, switch element 150 is actuated (switched from an open/non-conducting state to a closed/conducing state) at the start of a transient event by way of asserting trigger signal TS, whereby switch element 150 initiates a DC trigger current I_(T) that flows from battery PS through self-limiting resistive element 140 to ground. In one embodiment, self-limiting resistive element 140 includes a resistive portion 142 and a current-limiting portion 145 that are connected in series between battery PS and ground by way of a first terminal 141-1 and a second terminal 141-2, respectively, where at least one of resistive portion 142 and a current-limiting portion 145 are disposed on upper surface 111 over a localized (i.e., relatively small) region 111-21 of frangible glass substrate 110. One or both of resistive portion 142 and a current-limiting portion 145 are configured and operably thermally coupled to frangible glass substrate 110 such that self-limiting resistive element 140 generates heat at a rate that rapidly increases a localized temperature of localized region 110-21, and current limiting portion 145 is further configured to independently control (i.e., without requiring an externally-generated control signal) the flow of trigger current I_(T) by way of terminating the flow of trigger current I_(T) after localized region 110-21 receives a sufficiently large amount of the heat generated by self-limiting resistive element 140. In a preferred embodiment described below, current-limiting portion 145 is implemented by a fuse element configured to fail (melt and break) after conducting a predetermined amount of trigger current I_(T) (i.e., after a predetermined amount of heat is generated by self-limiting resistive element 140). In other embodiments, current-limiting portion 145 may be implemented by other elements/circuits configured to implement the described independent control over trigger current I_(T), such as a thermistor-based circuit configured to terminate the trigger current flow upon detecting a predetermined temperature level, or a timer-based circuit configured to terminate the trigger current flow a preset time after actuation of switch element 150.

FIG. 2 is a flow diagram showing a generalized method for triggering a disaggregation event on transient electronic device 100 (shown in FIG. 1), and FIGS. 3(A), 3(B), 3(C), 3(D), 3(E) and 3(F) are partial cross-sectional side views showing a portion of transient electronic device 100 before and during an exemplary disaggregation event.

Referring to the top of FIG. 2, transient electronic device 100(t0) (FIG. 1) is subjected to normal operation (i.e., functional circuitry 120 performs as intended). FIG. 3(A) is a simplified cross-section showing a portion of device 100(t0), where resistive portion 142 of self-limiting resistive element 140 is depicted as a metal structure disposed on upper surface 111 of the frangible glass substrate 110 over localized region 110-21, and current control portion 145 is depicted as a switch in a closed (i.e., conducting) state. At this time to, switch portion 150 is open (i.e., in a non-conducting state), so no current passes from battery PS through self-limiting resistive element 140, and thus no heat is generated by resistive portion 142. Accordingly, at time t0, a localized temperature T_(LR) in localized region 110-21 assumes the same (relatively low) initial temperature T₀ as that of surrounding adjacent regions 100-22 of frangible glass substrate 110.

Referring to block 205 near the top of FIG. 2, the transient event is initiated at a time t10, for example, when a trigger event signal is detected by a sensor (e.g., in response to a wirelessly transmitted light or RF signal), or unauthorized tampering is detected.

As indicated in block 210 (FIG. 2) and FIG. 3(B), at a time t11 (i.e., a moment after time t10), trigger signal TS (FIG. 1) is asserted and applied to the control terminal of switch element 150, whereby switch 150 is actuated to initiate the flow of trigger current I_(T) through resistive portion 142 and current control portion 145 to ground, whereby resistive portion 142 begins to generate heat H_(G) that is transmitted through upper surface 111 of frangible glass substrate 110 into localized region 110-21, thereby causing localized temperature T_(LR) to increase above initial temperature T₀.

As indicated in block 220 (FIG. 2) and FIG. 3(C), at a subsequent time t12, the continued flow of trigger current I_(T) causes resistive portion 142 to generate heat H_(G) at a rate that rapidly increases localized temperature T_(LR) toward a predetermined target temperature T₁. According to an aspect of the invention, this rapid temperature increase occurs at a rate that causes localized temperature T_(LR) to increase at a rate faster than the temperature in surrounding regions 110-22. That is, as indicated in FIG. 3(C), generated heat H_(G) from self-limiting resistive element 140 enters localized region 110-21 at a faster rate than dissipating heat H_(G) leaves localized region 110-21 into surrounding regions 110-22, thereby causing localized temperature T_(LR) of localized region 110-21 to rapidly increase from the initial temperature T₀ toward the higher target (first) temperature level T₁ while surrounding regions 110-22 remain at a substantially lower temperature.

As indicated in block 230 (FIG. 2) and FIG. 3(D), at time t13, current limiting portion 145 actuates to terminate the generation of heat H_(G) by way of generating an open circuit condition that terminates the flow of trigger current through resistive portion 142. According to another aspect of the invention, self-limiting resistive element is configured such that the termination of generated heat causes a rapid decrease of localized temperature TLR toward a lower (second) temperature T₂ by way of heat H_(D) dissipating out of localized region 110-21 into cooler surrounding regions 110-22.

As indicated in block 240 (FIG. 2) and FIG. 3(E), at time t14, the thermal pulse generated by the rapid temperature increase and rapid temperature decrease described above with reference to FIGS. 3(C) and 3(D) produces a stress profile in localized region 110-21 of frangible glass substrate 110 that is sufficient to produce an initial fracture force F_(o) in localized region 110-21.

As indicated in block 240 (FIG. 2) and FIG. 3(E), at time t14, the thermal pulse generated by the rapid temperature increase and rapid temperature decrease described above with reference to FIGS. 3(C) and 3(D) produces a stress profile in localized region 110-21 of frangible glass substrate 110 that has sufficient force to produce an initial fracture force F_(o) in localized region 110-21. Subsequently, as indicated in block 250 (FIG. 2) and FIG. 3(F), at time t15, the initial fracture force F_(o) produces propagating fracture forces Fp that radiate from localized region 110-21 throughout frangible glass substrate 110 and electronic elements 122, thereby causing the complete disaggregation depicted by device 100(t2) at the bottom of FIG. 1. Accordingly, by configuring trigger mechanism 130 such that it generate the thermal pulse in frangible glass substrate 110 as described above, and by configuring frangible glass substrate 110 such that it generates initial fracture force F₀ in localized region 110-21 in response to the applied thermal pulse, and such that initial fracture force F₀ causes propagating fracture forces F_(P) that result in complete disaggregation of frangible glass substrate 110 and electronic elements 122 in the manner described above, the present invention facilitates the construction of low-cost, highly reliable transient electronic devices 100 without the need for pulse shaping circuity and complicated addressing arrangements.

FIG. 4 is a time-temperature graph depicting changes to localized temperature T_(LR) that illustrate an exemplary thermal pulse generated by self-limiting resistive element 140 during the exemplary disaggregation event described above with reference to FIGS. 3(A) to 3(F), and FIG. 5 is a time-voltage/current graph illustrating exemplary operating characteristics of self-limiting resistive element 140 during the exemplary disaggregation event. As indicated in FIG. 4, localized temperature T_(LR) remains at initial temperature T₀ until time t11 (i.e., when the flow of trigger current is initiated as described above with reference to FIG. 3(B)). The subsequent rapid increase during the period from time t11 to time t13 is caused by generated heat H_(G) entering localized region 110-21 in the manner described above with reference to FIGS. 3(B) and 3(C). Self-limiting resistive element 140 is configured such that the peak (highest) value of localized temperature T_(LR) (i.e., high/first temperature T₁, which occurs at time t13, and is above the melting point temperature TMP of the glass forming frangible glass substrate 110. After self-limiting resistive element 140 actuates, heat begins dissipating from localized region 110-21 into surrounding regions 110-22 of frangible glass substrate 110, as depicted in FIG. 3(D), causing the subsequent rapid temperature decrease during the period from time t13 to time t14. At time t14, the stress profile caused by the rapid increase to temperature T₁ followed by the period of rapid cooling produces initial fracture force F₀ in localized region 110-21, as depicted in FIG. 3(E). Almost immediately after time t14, initial fracture force F₀ cause propagating fracture forces Fp that completely disaggregate device 100 in a very short amount of time, as depicted in FIG. 3(F), resulting is rapid cooling. As indicated in FIG. 5, the current through self-limiting resistive element 140 jumps to a maximum value (e.g., approximately 5 Amps) at time t10, when switch 150 actuates, slowly decreases as resistive portion 142 gradually heats up, and then drops to zero at time t14 when current control element 145 actuates. In contrast, the voltage across self-limiting resistive element 140 gradually increases from 0V at time t10 to an intermediate value (e.g., approximately 4 Volts) as resistive portion 142 gradually heats up, and then jumps to a maximum value (e.g., approximately 9 Volts) at time t14 when current control element 145 actuates.

FIG. 6 is a perspective view showing a portion of a transient electronic device 100A having a self-limiting resistive element 140A comprising a fuse-type current-limiting portion 145A connected in series between two (first and second) resistor structures (resistive portions) 142A-1 and 142A-2, which are respectively connected to opposing (first and second) terminals 142A-1/2 and disposed over localized region 110-21 in the manner described above. As in the embodiment shown in FIG. 1, when operably connected together, terminal 141A-1 is coupled by way of a switch element (not shown) to the direct current (DC) power source and terminal 141A-2 is connected to a suitable ground structure. In a preferred embodiment, resistor structures 142A-1 and 142A-2 and fuse-type current-limiting portion 145A are produced by depositing a single (common) resistive material (e.g., one or more of magnesium, copper, tungsten, aluminum, molybdenum and chrome) directly onto upper surface 111A of frangible glass substrate 110A. The critical issue associated with the selection of the resistive material is that it is sufficiently conductive and adheres to upper surface 111A of frangible glass substrate 110A well enough that heat generated by resistive portions 142A-1 and 142A-2 and current-limiting portion 145A is transferred efficiently into localized region 110-21 during the rapid heating process of a thermal pulse. The resistive material is printed, etched or otherwise patterned such that resistor structures 142A-1 and 142A-2 respectively comprise relatively large structures 142A-1/2, and fuse-type current-limiting portion 145A comprises a relatively narrow thin structure configured to function as a fuse element. That is, when a sufficiently large current passes between terminals 141A-1 and 141A-2, resistor structures 142A-1 and 142A-2 and current-limiting portion 145A undergo resistive heating, but its relatively narrow cross-section causes current-limiting portion 145A to produce a higher temperature, which in the preferred embodiment is sufficient to cause melting and breakage when subjected to a suitable trigger current. Implementing current-limiting portion 145A using a fuse-type current-limiting portion (fuse element) provides a low-cost, simple and highly reliable structure for independently controlling the amount of heat generated by self-limiting resistive element 140A during the rapid heating process of a thermal pulse, and reliably produces an open circuit condition (i.e., by way of melting/breaking) that terminates flow of trigger current through resistor structures 142A-1 and 142A-2 at the start of the rapid cooling portion of the thermal pulse.

FIGS. 7(A) and 7(B) are top plan views showing self-limiting resistive elements 140B-1 and 140B-2 including bowtie-type patterned metal layer structure disposed directly on upper surface 111B of frangible glass substrate 110B, with each resistive element including a fuse element connected between downward-tapered sections of two resistor structures. For example, self-limiting resistive element 140B-1 includes a first resistor structure 142B-11 including a downward-tapered section 146B-11, a second resistor structure 142B-12 including a downward-tapered section 146B-12, and a fuse element 145B-1 connected between tapered ends 147B-11 and 147B-12 of downward-tapered sections 146B-11 and 146B-12, respectively. Similarly, self-limiting resistive element 140B-2 includes a first resistor structure 142B-21 including a downward-tapered section 146B-21, a second resistor structure 142B-22 including a downward-tapered section 146B-22, and a fuse element 145B-2 connected between tapered ends 147B-21 and 147B-22. In each case, fuse elements 145B-1 and 145B-2 comprise narrow neck structures having widths W1 that are configured to melt and break when subjected to a trigger current. In a preferred embodiment, width W1 is greater than a thickness frangible glass substrate 110B (e.g., thickness T shown in FIG. 6). For example, in a practical embodiment using a 0.25 mm thick frangible glass substrate, fuse elements 145B-1 and 145B-2 have widths W1 of at least 0.3 mm. In contrast, the resistor structures of both self-limiting resistive elements (e.g., resistor structure 146B-11) can have almost any width (size) W2, although larger resistors require more power and energy from the power source.

As depicted by the alternative embodiments shown in FIGS. 7(A) and 7(B), different fuse configurations may be utilized to generate desired thermal pulse characteristics. Specifically, self-limiting resistive elements 140B-1 and 140B-2 differ in that fuse element 145B-1 comprises a straight rectangular structure extending between tapered ends 147B-11 and 147B-12, and fuse element 145B-2 comprises an S-shaped structure extending between tapered ends 147B-21 and 147B-22. These different fuse arrangements provide different benefits, by allowing control over the applied energy and power by tuning the resistance of the trigger as well as tuning the area over which the heat is applied which can impact the time to fragment.

FIG. 8 is a circuit diagram showing a partial transient electronic device 100C including a remotely (wirelessly) controllable trigger mechanism 130C including a sensor 160C configured to detect a transmitted wave signal WS (e.g., a light wave signal, a radio frequency (RF), or an acoustic/sound signal), and configured to then generate the electronic trigger signal TS used to actuate the switch element. As in the previous embodiment, trigger mechanism 130C includes a self-limiting resistive element 140C and a switch element 150C, where self-limiting resistive element 140C includes a fuse element 145C connected between a first resistor structure 142C-1 and a second resistor structure 142C-2. In an exemplary embodiment, switch element 150C is implemented using a silicon controlled rectifier, wave signal WS is an optical signal, and sensor 160C is a photodiode (or other light wave sensor) operably coupled to switch element 150C and configured to activate in response to optical signal WS such that current through photodiode 160C actuates switch element 150C by way of utilizing the current to cause the silicon controlled rectifier to latch, which in turn couples battery PS across self-limiting resistor element 140C, ultimately causing disaggregation (fragmentation) of the glass substrate and any included electronics in the manner described above. While remote actuation of the transient electronic device is achieved using an optical signal in this example, RF signals and acoustic wave signals may be utilized by replacing photodiode 160C with a radio-frequency wave sensor or an acoustic wave sensor. Moreover, other physical and chemical stimuli may also be utilized in conjunction with an appropriate sensor to initiate a fragmentation sequence. Similarly, while latching is achieved using silicon controlled rectifier 150C in the practical embodiment, other switch elements may also be utilized, such as a latch circuit comprising a single MOSFET transistor or a MOSFET-based multiple-element circuit.

FIGS. 9(A) and 9(B) show simplified transient electronic devices 100D-1 and 100D-2 according to alternative practical embodiments in which sensor and/or switch elements are either fabricated or mounted directly on the “host” frangible glass substrate, or fabricated concurrently with the electronic elements on a semiconductor layer. For example, FIG. 9(A) shows a device 110D-1 in which a sensor 160D-1 (e.g., a photodiode) and a switch element 150D-1 (e.g., a silicon controlled rectifier) are implemented by suitable materials printed or patterned directly onto surface 111D of frangible glass substrate 110D. Alternatively, as shown in FIG. 9(B), device 110D-2 includes a sensor 160D-2 and a switch element 150D-2 fabricated using CMOS fabrication techniques on a semiconductor structure 121D-2 (e.g., IC chip or SOI layer) on which the electronic elements 122D are formed. In both embodiments, self-limiting resistive elements 140D-1 and 140D-2 are formed directly on substrates 1 In another embodiment (not shown), one or both of the sensor and switch element may be disposed on a host PC board and connected by way of suitable conductive connection to the

Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention. For example, although self-limiting resistive elements having bowtie-type arrangements are described above, other arrangements may be used. 

The invention claimed is:
 1. A transient electronic device, comprising: a frangible glass substrate; and a trigger mechanism comprising: a self-limiting resistive element comprising a material disposed over a localized region of the frangible glass substrate, the self-limiting resistive element configured such that: during a time period when a current from a power source flows through the self-limiting resistive element, heat generated by the self-limiting resistive element increases a localized temperature of the localized region from an initial temperature toward a first temperature level; and a current-limiting portion of the self-limiting resistive element is configured to control the flow of a trigger current such that the trigger current flow is terminated when the localized temperature of the localized region increases to the first temperature level, thereby causing the localized temperature to decrease from the first temperature level to a second temperature level; wherein the frangible glass substrate is configured such that the localized temperature increase and the localized temperature decrease causes the frangible glass substrate to fracture.
 2. The device of claim 1, wherein the current-limiting portion comprises an electronic circuit configured to terminate the flow of the trigger current.
 3. The device of claim 1, wherein the current-limiting portion comprises a thermistor-based circuit configured to terminate the flow of the trigger current in response to detecting the first temperature level.
 4. The device of claim 1, wherein the current-limiting portion comprises a timer-based circuit configured to terminate the flow of the trigger current after a preset time following initiation of the flow of the trigger current.
 5. The device of claim 1, wherein the self-limiting resistive element comprises resistive structures, and the resistive structures and the current-limiting portion comprise two or more different materials with different melting points and different electrical conductivity.
 6. The device of claim 1, wherein: the self-limiting resistive element comprises resistive structures; the current-limiting portion is rectangular in shape; and a width of the current-limiting portion is different from a widest dimension of the resistive structures.
 7. The device of claim 1, wherein the current-limiting portion comprises a fuse element.
 8. The device of claim 7, wherein the fuse element has a meandering shape.
 9. The device of claim 1, wherein: the triggering mechanism further comprises a sensor configured to detect a predetermined light, temperature, acoustic or radio wave signal; and the triggering mechanism is configured to activate the power source in response to detecting the predetermined signal.
 10. A method of fracturing a transient electronic device, comprising: during a time period when a current from a power source flows through a self-limiting resistive element disposed over a localized region of a frangible glass substrate, generating heat by the self-limiting resistive element to increase a localized temperature of the localized region from an initial temperature toward a first temperature level; controlling the flow of a trigger current by a current-limiting portion of the self-limiting resistive element such that the trigger current flow is terminated when the localized temperature of the localized region increases to the first temperature level, thereby causing the localized temperature to decrease from the first temperature level to a second temperature level; and fracturing the frangible glass substrate in response to the increase and decrease in the localized temperature.
 11. The method of claim 10, wherein: the current-limiting portion comprises an electronic circuit; and the method further comprises terminating the flow of the trigger current by the electronic circuit.
 12. The method of claim 10, wherein: the current-limiting portion comprises a thermistor-based circuit; and the method further comprises terminating the flow of the trigger current by the thermistor-based circuit in response to detecting the first temperature level.
 13. The method of claim 10, wherein: the current-limiting portion comprises a timer-based circuit; and the method further comprises terminating the flow of the trigger current by the timer-based circuit after a preset time following initiation of the flow of the trigger current.
 14. The method of claim 10, wherein: the current-limiting portion comprises a fuse element; and the method further comprises terminating the flow of the trigger current in response to melting or breaking of the fuse element.
 15. The method of claim 14, wherein the fuse element has a meandering shape.
 16. The method of claim 10, wherein the triggering mechanism further comprises a sensor, and the method further comprises: detecting, by the sensor, a predetermined light, temperature, acoustic or radio wave signal; and activating the power source in response to detecting the predetermined signal.
 17. A transient electronic device, comprising: a frangible glass substrate; one or more electronic elements disposed on the frangible glass substrate; and a trigger mechanism comprising: a self-limiting resistive element comprising a material disposed over a localized region of the frangible glass substrate, the self-limiting resistive element configured such that: during a time period when a current from a power source flows through the self-limiting resistive element, heat generated by the self-limiting resistive element increases a localized temperature of the localized region from an initial temperature toward a first temperature level; and a current-limiting portion of the self-limiting resistive element is configured to control the flow of a trigger current such that the trigger current flow is terminated when the localized temperature of the localized region increases to the first temperature level, thereby causing the localized temperature to decrease from the first temperature level to a second temperature level; wherein the frangible glass substrate is configured such that the localized temperature increase and the localized temperature decrease causes the frangible glass substrate to fracture.
 18. The device of claim 17, wherein the current-limiting portion comprises an electronic circuit configured to terminate the flow of the trigger current.
 19. The device of claim 17, wherein the current-limiting portion comprises a thermistor-based circuit configured to terminate the flow of the trigger current in response to detecting the first temperature level.
 20. The device of claim 17, wherein the current-limiting portion comprises a timer-based circuit configured to terminate the flow of the trigger current after a preset time following initiation of the flow of the trigger current.
 21. The device of claim 17, wherein the current-limiting portion comprises a fuse element.
 22. The device of claim 21, wherein the fuse element has a meandering shape. 